DISTRIBUTION OF DESCENDING PROJECTIONS FROM PRIMARY AUDITORY NEOCORTEX TO INFERIOR COLLICULUS MIMICS THE TOPOGRAPHY OF INTRACOLLICULAR PROJECTIONS

To ascertain whether the auditory neocortex also innervates the centra l nucleus of the inferior colliculus (CNIC) and not only its dorsal (D CIC) and external (ECIC) cortices. the anterograde tracers Phaseolus v ulgaris-leucoagglutinin (PHA-L) and biotinylated dextran (ED) were inj ected into the primary auditory neocortex of albino rats (Te1), and la beled corticocollicular fibers were studied via light and electron mic roscopy. Axons from discrete regions of Te1 form two rostrocaudally or iented laminar plexuses of terminal fibers in the ipsilateral inferior colliculus (IC) and one in the contralateral IC. The first ipsilatera l plexus, located in the medial half of the IC, has a dorsomedial to v entrolateral orientation, parallel to the isofrequency planes of the I C; is continuous through the CNIC and DCIC and extends into the rostra l ECIC. The second plexus is located in the deep layers of the lateral ECIC. These two plexuses meet caudally and ventrally, at the border b etween the CNIC and the lateral ECIC. The plexus in the contralateral IC is less dense and shorter than the two ipsilateral plexuses and is symmetric to the medial plexus. The thickness of the three plexuses is correlated with the size of the injection site, and their mediolatera l and dorsoventral positions change as the injection site in Te1 is di splaced rostrocaudally, with more caudal injections resulting in more dorsolateral medial plexuses and more dorsomedial lateral plexuses. Fu rthermore, the ventromedial border of the IC receives nontopographic, convergent projections from wide regions of rostral portions of Te1. T he distribution of these corticocollicular plexuses mimics the topogra phy of previously described intracollicular fibers. Electron microscop y shows that, in all three subdivisions of the ipsilateral IC, cortico collicular fibers form small boutons with features generally associate d with excitatory transmission; i.e., they contain round synaptic vesi cles and form asymmetric synapses with thin dendritic shafts and spine s. These results demonstrate that the auditory corticocollicular proje ctions innervate more extensive regions of the IC than were previously observed. Although peripheral regions receive the densest projection, the entire IC appears to be the target of corticofugal input. (C) 199 6 Wiley-Liss, Inc.